The present invention relates to an electron-beam exposure system and a method applied therein, and particularly, for example, to those applied in a cell projection lithography wherein a desired semiconductor device pattern is divided into several cells and each of the cells is exposed shot by shot.
Recently, high throughput has been pursued in fabrication processes of semiconductor devices, such as memory chips, by using an electron-beam (hereafter abbreviated (EB)) lithography process for printing a fine pattern on a semiconductor wafer. For enabling the high throughput, the cell projection lithography is used, making use of EB masks where desired cell patterns are prepared.
In cell projection lithography, open space of each cell pattern of the EB mask differs according to each pattern design to be printed on the cell in question, and consequently, current intensity of the electron beam attaining through the EB mask to the object material, such as a semiconductor wafer, varies according to the open space of the selected cell pattern.
This variation of the EB current intensity affects focusing of the electron beam. FIG. 8 is a schematic diagram illustrating the focus deviation according to the EB current intensity.
The larger the open space of the cell pattern is, the higher the EB current intensity becomes. When a cell pattern A of an EB mask 701 has a large open space and cell patterns B and C have a medium and a small open space, respectively, the EB current intensity of an electron beam 702 passing through the cell pattern A is higher than that passing through the cell pattern B and forces called the Coulomb effect, operating among electrons in the electron beam, become stronger, which makes a focusing beam 703a, corresponding to the cell pattern A focused on a wafer 704, wider than a focusing beam 703b, corresponding to the cell pattern B, thus resulting in out of focus printing on surface of the wafer 704, as shown in FIG. 8.
On the other hand, the EB current intensity of the electron beam 702 passing through the cell pattern C becomes lower than that passing through the cell pattern B, and a focusing beam 703c, due to corresponding to the cell pattern C, is narrower than 703b by the Coulomb effect, resulting also in out of focus printing.
Therefore, each time a different cell pattern is selected, the lens system is re-adjusted conventionally, to make the electron beam focus correctly. That is, by measuring beforehand the EB current intensity reaching the wafer for each cell pattern of the EB mask, dynamic focus lenses are controlled for each cell pattern so that the electron beam may focus just on the wafer surface.
However, as reported in "Coulomb Interaction Effect in Cell Projection Lithography" by Yamashita et al., Jpn. J. Appl. Phys. Vol. 34 (1995) pp. 6684-6688, there is a problem in that the pattern resolution becomes degraded according to increase of the EB current because of aberration, for example, chromatic aberration, which becomes large along with the EB current and cannot be compensated for with focus control by way of lenses.
FIG. 9 is a characteristic chart illustrating a relationship between pattern resolution limit, represented by Lines-and-Spaces (L/S) size, and the EB current. It can be seen from FIG. 9 that the pattern resolution depends on the EB current intensity and maximum EB current is limited according to necessary resolution.
To eliminate this problem, there is a method to limit current intensity of electron beams passing through cell patterns by preparing an area (size) of each of the cell patterns to have the same open space, considering each pattern design to be printed.
Although it is not intended to be applied to the cell projection lithography, there is another method proposed for maintaining the EB current intensity within a fixed value by controlling emission current of the electron-beam source, in a Japanese patent application laid open as a Provisional Publication No. 88737/'88, wherein the emission current is feed-back-controlled by a monitoring current flowing through a filter provided on a beam axis of the electron-beam source.
FIG. 10 is a schematic diagram illustrating application of the prior method to the cell projection lithography.
An electron beam 902 radiated from an EB source 901 passes through a cell pattern of an EB mask 903 and is irradiated on a wafer 905 through a filter 904 provided for detecting current flowing through it in proportion with the EB current. When the electron beam 902 passes a cell pattern A having a large open space, the detected current should become large, with which the emission current of the EB source 901 is decreased to give the same EB current intensity.
Such as in the prior arts above described, by controlling projection cell sizes or the emission current, current intensity of the electron beams irradiated on the object material can be controlled within a fixed value, without needing to re-adjust lens system for each of the cell patterns.
However, in the method of controlling projection cell sizes according to the rate of open space of each cell pattern, a fairly large number of shots may be needed because of cell patterns are divided for limiting open space of a light pattern, resulting in degradation of the throughput not to mention the intricate processes required by various sizes of cell patterns included in a mask pattern.
As to the method of controlling the emission current for each cell pattern, the emission current should be changed each time a cell pattern having different open space is selected. A certain standby time is needed until the beam current is stabilized, resulting also in degradation of the throughput per time, even if each EB current monitoring process may economized by beforehand measuring the EB current for each cell pattern.